Device for processing light/optical radiation, method and system for designing such a device

ABSTRACT

A device is provided for processing a light/optical radiation including at least two reflective optical elements defining a multi-pass cavity so that at least one of the optical elements reflects the light radiation at least twice, at at least two different reflection positions, and including at least one element, called a corrective element, having at least one position, called a corrective position, producing a reflection or a transmission of the optical radiation, and the surface of which is irregular so that the spatial phase profile of the corrective position has a different phase shift for several different reflection/transmission points of the corrective position.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/113,761, filed Jul. 22, 2016, which is a national phase entry under35 U.S.C. § 371 of International Patent Application PCT/EP2015/050711,filed Jan. 15, 2015, designating the U.S. and published as InternationalPatent Publication WO 2015/113831 A1 on Aug. 6, 2015, which claims thebenefit under Article 8 of the Patent Cooperation Treaty to FrenchPatent Application Serial No. 1450715, filed Jan. 30, 2014.

TECHNICAL FIELD

This disclosure relates to a device for processing a light/opticalradiation, in particular by a succession of propagations andmodifications of the spatial phase of the light radiation. It alsorelates to a method and a system for designing such a device.

The field of the disclosure is the field of processing optical radiationand, in particular, the field of processing optical radiation requiringa succession of propagations of the light radiation.

BACKGROUND

Document WO 2012/085046 A1 describes a system for correcting the effectof a diffusing medium on optical radiation having propagated in thisdiffusing medium, or in order to transform arbitrarily the spatialproperties of optical radiation. It relates to a system for processingthe light radiation.

The system described in this document comprises a plurality of opticalmeans (phase plates or spatial phase modulators) separate from eachother, the phase profile of which can be adjusted individually during anoptimization step, and which allows each to modify the spatial phase ofthe light radiation which passes through them or which is reflected onthem. It is the sequence of these spatial phase modifications of thelight radiation separated by propagations that makes it possible, ingeneral terms, to process the light radiation.

More generally, the optical systems provided for processing a lightradiation comprise a plurality of optical means separate from eachother, each carrying out a given processing on the optical radiation.

These optical systems, which comprise several optical means making itpossible to modify the phase profile of the radiation, and in which theoptical radiation carries out a succession of propagations, have a majordrawback. In these systems, the positioning of the optical means withrespect to each other and with respect to the light radiation must bevery accurate, typically of the order of a micron, which can bedifficult to achieve, time-consuming to assemble, and increase therequirements for rigidity of the mounting. Poor positioning of anoptical element causes degradation of the processing carried out on theoptical radiation. Thus, the optical radiation at the output of theprocessing device then degrades. This degradation can take the form, forexample, of a loss of intensity or unwanted spatial deformation of theoutput radiation.

The purpose of the disclosure is to overcome the aforementioneddrawbacks.

A further purpose of the disclosure is to propose a device forprocessing optical radiation that is easier to construct.

Yet a further purpose of the disclosure is to propose a device forprocessing optical radiation that is quicker to construct.

Finally, a further purpose of the disclosure is to propose a device forprocessing optical radiation that is more resistant to impacts andvibrations.

BRIEF SUMMARY

The disclosure makes it possible to achieve at least one of theaforementioned purposes by a device for processing a light radiationcomprising at least two reflective optical elements defining amulti-pass cavity so that at least one of the optical elements reflectsthe light radiation at least twice, in particular at at least twodifferent reflection positions, characterized in that it comprises atleast one element, called a corrective element, having at least oneposition, a called corrective position, producing a reflection or atransmission of the optical radiation, the surface of which is irregularso that the spatial phase profile of the corrective position has adifferent phase shift for several different reflection/transmissionpoints of the corrective position.

In other words, a corrective position modifies the phases of at leasttwo spatial components of light radiation differently.

Thus, the device, according to the disclosure, processes a light oroptical radiation by a multi-pass cavity defined in a fixed manner andby a low number of optical elements, in which the light radiation isreflected a plurality of times. The spatial phase of the light radiationis modified during at least one reflection or transmission on at leastone corrective element.

As a result, the device, according to the disclosure, carries out withone and the same fixed light, one or more spatial phase modifications ofthe light radiation.

More generally, the device, according to the disclosure, makes itpossible to carry out a plurality of spatial phase modifications of thelight radiation with one and the same fixed optical element, beingcapable of presenting different phase profiles for different reflectionor transmission positions, while the systems of the state of the artenvisage using as many optical elements as there are modifications ofthe spatial phase of the light radiation.

The device, according to the disclosure, is easier to install, to use,and quicker to configure, as the number of optical elements to bepositioned in relation to each other and with respect to the opticalradiation is lower. Moreover, the low number of optical elements makesthe device, according to the disclosure, more cost-effective tomanufacture and allows the rigidity and solidity of the final system tobe improved.

Of course, as this is a multi-pass cavity, the optical radiation isreflected on each reflective element in turn. In other words, tworeflections of the optical radiation on one of the reflective opticalelements defining the multi-pass cavity are separated by one reflectionon the other one of the reflective optical elements defining themulti-pass cavity.

In this disclosure, the “spatial phase of the radiation” is defined bythe phases of all of the spatial components of the radiation.

In addition, the “spatial phase profile of a corrective position”(reflection or transmission) is defined by all of the spatial phaseshifts (modifications of the spatial phase) contributed by all of thereflection or transmission points of the corrective position on thedifferent spatial components of the light radiation during one and thesame reflection or transmission of the radiation on the correctiveposition. The phase profile can be very simple in the case of areflection on a plane mirror.

Moreover, according to the disclosure, a corrective position can produceeither a reflection of the radiation or a transmission of the radiation.

Each reflective optical element (from the at least two reflectiveoptical elements defining a multi-pass cavity) reflects the lightradiation preferably a plurality of times (preferably at least fourtimes, preferably at least six times).

According to the disclosure, the irregularities of a corrective positioncan be obtained:

-   -   by modifying the depths of the reflection or transmission        surface by etching of the surface or deposition of a resin on        the surface, in which case the depth of reflection or of        transmission is modified, and/or    -   by depositing on, or by producing, the corrective position with        a material modulating the phase of the spatial components of the        radiation, in which case the depth of reflection or of        transmission is not modified, for example, with liquid crystals.

According to a preferred but non-limitative embodiment, theirregularities of a corrective position can have spatial structures atleast 5 times smaller than the total size of the corrective position.

Advantageously, at least one optical corrective element can be formed byone of the reflective optical elements defining the multi-pass cavity.In this case, the number of optical elements of the device, according tothe disclosure, is reduced, as one and the same optical element is bothcorrective and defines the multi-pass cavity.

According to a particular embodiment, the device, according to thedisclosure, can comprise a single corrective element corresponding toone of the reflective optical elements defining the multi-pass cavity.Thus, the device, according to the disclosure, is simpler to configureand less expensive as it requires a single optical element modifying thespatial phase of the radiation that also defines the multi-pass cavity.

According to another particular embodiment, the device, according to thedisclosure, can comprise two corrective elements correspondingrespectively to the reflective optical elements defining the multi-passcavity. In this case, each reflective optical element defining themulti-pass cavity comprises at least one corrective reflection positionmodifying the spatial phase of the radiation.

In this version, the device, according to the disclosure, makes itpossible to carry out a modification of the spatial phase of the lightradiation by means of the two reflective elements defining themulti-pass cavity. Thus, it is possible to carry out more completeprocessing of the light radiation with a smaller number of reflectionsin the multi-pass cavity. As a result, in this version, the device,according to the disclosure, carries out processing of the lightradiation while minimizing the number of optical elements required.

According to the disclosure, at least one corrective element can beplaced in the multi-pass cavity and be different from the reflectiveoptical elements defining the multi-pass cavity.

In this case, at least one of the reflective optical elements definingthe multi-pass cavity can also be corrective. Alternatively, it ispossible for the reflective optical elements defining the multi-passcavity not to be corrective.

In a preferred version of the device, according to the disclosure, atleast two, in particular all, the reflection positions of at least onecorrective element are corrective. Thus, each corrective position has anirregular reflection or transmission surface so that the spatial phaseprofile of each corrective position has a different phase shift forseveral points of reflection or of transmission of the correctiveposition.

In this version, the device, according to the disclosure, makes itpossible to carry out a modification of the spatial phase of the lightradiation during several, in particular all, the reflections ortransmissions, on the corrective element. Thus, it is possible to carryout more complete and more complex processing of the light radiation inthe multi-pass cavity.

Advantageously, at least two corrective positions of one and the samecorrective element have different phase profiles. Thus, the device makesit possible to modify differently the spatial phase of the radiationduring reflections or transmissions on these two corrective positions ofone and the same corrective optical element.

Alternatively or in addition, at least two corrective positions of oneand the same corrective element have identical phase profiles. Thus, thedevice makes it possible to modify identically the spatial phase of theradiation during reflections or transmission on these two correctivepositions of one and the same corrective optical element.

Moreover, at least two corrective positions of two different opticalcorrective elements may have identical or different spatial phaseprofiles.

According to an embodiment, at least one optical corrective element canbe a phase plate.

Advantageously, at least one corrective optical element can be a phaseplate having at least two different spatial phase profiles for at leasttwo corrective positions.

In this case, the phase plate covers at least two different correctivepositions. Each part of the phase plate corresponding to a correctiveposition has a desired spatial phase profile for this correctiveposition, which can be different from the desired spatial phase profilefor another corrective position covered by the phase plate. Thus, thesingle phase plate covering several corrective positions comprises atleast two parts having different spatial phase profiles. In thisembodiment, the construction of the device, according to the disclosure,is facilitated because one single phase plate is manipulated andpositioned in the multi-pass cavity.

According to an embodiment, the phase plate can be an etched mirror, butcan also be a resin deposited on a substrate.

Advantageously, at least one corrective optical element can be a spatialphase modulator having at least two different spatial phase profiles forat least two corrective positions.

In this case, the spatial phase modulator covers at least two differentcorrective positions. Each part of the spatial phase modulatorcorresponding to a corrective position has a desired spatial phaseprofile for this corrective position, which can be different from thedesired spatial phase profile for another corrective position covered bythe phase plate. Thus, the single spatial phase modulator coveringseveral corrective positions comprises at least two parts havingdifferent spatial phase profiles. In this embodiment, the cost of thedevice, according to the disclosure, is reduced, as one single spatialphase modulator is required for the device.

According to an embodiment, the spatial phase modulator can be a mirrordeformed by actuators, but can also be a liquid crystal array, thebirefringence properties of which are controlled by an array ofelectrodes.

Moreover, at least one reflective element defining the multi-pass cavitycan comprise a through-hole making it possible to inject into themulti-pass cavity the optical radiation to be processed and/or to outputthe optical radiation from the multi-pass cavity after processing.

In a preferred, non-limitative embodiment of the device, according tothe disclosure, one of the reflective optical elements can have a planarreflective surface and the other one of the reflective optical elementscan have a curved reflective surface.

In particular, the planar surface can be a phase plate having one ormore corrective reflection positions, the curved surface not applyingany modification of the particular spatial phase of the light radiationother than the curvature mentioned.

In a preferred but non-limitative version of the device, according tothe disclosure, the reflective optical elements defining the multi-passcavity can be positioned in two directions perpendicular to each other.In this case, the device, according to the disclosure, can moreovercomprise a mirror, called an intermediate mirror, placed facing thereflective optical elements at an angle of 45° with respect to thedirection of each of the reflective optical elements, and reflecting 99%of the light radiation.

The intermediate mirror can be a corrective element or not.

In this non-limitative version, the device, according to the disclosure,is easier to configure, as it is easy to observe each of the reflectiveoptical elements individually by observing, for example, the 1% of thelight radiation that passes through the intermediate mirror.

In this version of the device, according to the disclosure, themulti-pass cavity is called “angled.”

According to another aspect of the disclosure, a method is proposed forprocessing an optical radiation implementing a device according to thedisclosure.

According to another aspect of the disclosure, a system is proposed forprocessing an optical radiation, the system comprising:

-   -   a device according to the disclosure;    -   a means for injecting the radiation into the corrective device,        and    -   a means for collecting the radiation at the output of the        corrective device.

According to another aspect of the disclosure, a method is proposed fordesigning a device according to the disclosure, the method comprisingthe following steps:

-   -   propagating the optical radiation to be processed and a        radiation, called a reference radiation, in a multi-pass cavity        defined between two reflective optical elements so that at least        one of the optical elements reflects the optical radiation at        least twice, in particular in at least two different reflection        positions;        -   optimizing an interference between the radiations, the            optimization comprising at least one iteration of the            following steps, carried out for at least one corrective            position, on at least one corrective element:        -   determining a parameter relating to an interference between            the radiations, for example, at the level of at least one            corrective position, and        -   modifying the spatial phase profile of the at least one            corrective position; and        -   configuring, at the level of at least one corrective            position, the spatial phase profile, determined during the            optimization step, and providing the optimized interference.

Each of the radiations, one to be processed and one reference, can be:

-   -   a radiation effectively propagated in the multi-pass cavity, the        reference radiation propagating in a direction of propagation        opposite to the direction of propagation of the optical        radiation to be processed; or    -   a virtual radiation represented by a set of digital data. In        this second case, the propagations of the radiations reflect the        properties measured in the multi-pass cavity.

In the case in which the radiation to be processed and/or the referenceradiation is virtual, the method can comprise a step for collecting dataon the geometry of the multi-pass cavity by measuring the positions andthe amplitude distribution of the reflections or transmissions of thereference radiation and/or the radiation to be processed, on thedifferent optical elements in the absence of phase profile. Such ameasurement can be carried out by a measurement means, such as acharge-coupled device (CCD) camera, placed facing or behind thereflection or transmission position.

In the case in which the multi-pass cavity is angled, the intermediatemirror can be partially reflective and the measurement means, forexample, the CCD camera, can be placed behind the intermediate mirrorfacing the corrective position and, more generally, facing thereflecting or transmitting element on which the corrective reflectionposition is located.

The radiation to be processed and/or the reference radiation can bepropagated in the multi-pass cavity by simulation on computerized means,the simulation taking account of the characteristics of the cavity,namely optical and physical characteristics of the optical elementsdefining the cavity, the length of the cavity, the relative angles ofthe different optical elements, etc.; characteristics calculated byusing among others the information captured during the step ofcollection of data relating to the geometry of the multi-pass cavity.

The propagation through the cavity of the virtual reference radiationand the virtual radiation to be processed can be carried out in order toprovide the characteristics of the reference radiation and the radiationto be processed at the level of each of the corrective positions withinthe cavity, namely the intensity and phase shift of each spatialcomponent of the reference radiation and the radiation to be processedat the level of each of the corrective positions, so as to determine thecorrelation parameter of these two radiations as described above.

In addition, in the case in which the processing to be carried out isthe correction of the effect of a diffusing medium on an opticalradiation having passed through this medium, the radiation to beprocessed is obtained at the outlet of the diffusing medium and thereference radiation can advantageously be identical to the radiation tobe processed before the radiation to be processed passed through thediffusing medium. In other words, the reference radiation can beidentical to the radiation for processing, before the latter passesthrough the diffusing medium.

The correlation parameter can be determined at each reflection ortransmission corrective position, in the multi-pass cavity, or solely atthe level of a part only of the corrective positions in the cavity. Forexample, the correlation parameter can be measured only at the level ofthe corrective positions provided in order to apply processing to theoptical radiation to be processed. This correlation parameter can be thespatial phase difference between the radiation to be processed and thereference radiation.

Alternatively, or in addition, the correlation parameter can be measuredat the outlet of the multi-pass cavity.

Document WO 2012/085046 A1 comprises further details concerning theinterference and optimization measurements.

The method for configuration of a corrective position (reflection ortransmission) can comprise a step of measuring the characteristics ofthe radiation to be processed and the reference radiation at the levelof this corrective position. Such a measurement can be carried out by ameasurement means, such as a CCD camera, placed facing or behind thecorrective position.

In the case in which the multi-pass cavity is angled, the intermediatemirror can be partially reflective and the measurement means, forexample, the CCD camera, can be placed behind the intermediate mirrorfacing the corrective position and, more generally, facing thereflecting/transmitting element on which the corrective position islocated.

For a given corrective position, the step of configuring the device forprocessing the light radiation, called optimized, determined during theoptimization step, and providing the desired processing, can comprisethe following steps:

-   -   production of a phase plate comprising the optimized phase        profile or profiles,    -   positioning the phase plate at the corrective position(s).

The phase plate can be either an individual phase plate for a correctiveposition, or a phase plate common to several corrective positions andcomprising different optimized phase profiles on different regions ofits surface each corresponding to one corrective position.

According to yet another aspect of the disclosure, a system is proposedfor designing a device according to the disclosure, the systemcomprising:

-   -   at least one means for propagating the radiation to be processed        and a radiation, called reference radiation, in a multi-pass        cavity defined between two reflective optical elements so that        at least one of the optical elements reflects the optical        radiation at least twice, in particular in at least two        different reflection positions;    -   means for optimizing an interference between the radiations, the        optimization being carried out either by digital or optical        means, and comprising at least one iteration of the following        steps, carried out for at least one corrective position, on at        least one corrective element:        -   determining a parameter relating to an interference between            the radiations, for example, at the level of at least one            corrective position, and        -   modifying the spatial phase profile of the at least one            corrective position; and    -   at least one means for configuring, at the level of at least one        corrective position, the phase profile, determined during the        optimization step, and providing the optimized interface.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and characteristics will become apparent on examinationof the detailed description of embodiments, which are in no waylimitative, and the accompanying diagrams, in which:

FIG. 1 is a diagrammatic representation of a non-limitative embodimentof a device according to the disclosure;

FIG. 2 is a diagrammatic representation of another non-limitativeembodiment of a device according to the disclosure;

FIG. 3 is a representation in the form of a diagram of a non-limitativeexample of a method, according to the disclosure, for designing a deviceaccording to the disclosure; and

FIG. 4 is a diagrammatic representation of a non-limitative example of asystem according to the disclosure for designing a device according tothe disclosure.

DETAILED DESCRIPTION

It is well understood that the embodiments that will be describedhereinafter are in no way limitative. It is possible to envisagevariants of the disclosure comprising only a selection of thecharacteristics described hereinafter, in isolation from the othercharacteristics described, if this selection of characteristics issufficient to confer a technical advantage or to differentiate thedisclosure with respect to the state of the art. This selectioncomprises at least one, preferably functional, characteristic withoutstructural details, or with only a part of the structural details ifthis part alone is sufficient to confer a technical advantage or todifferentiate the disclosure with respect to the state of the prior art.

In particular, all the variants and all the embodiments described can becombined together if there is no objection to this combination from atechnical point of view.

In the figures, elements common to several figures retain the samereference.

FIG. 1 is a diagrammatic representation of a first non-limitativeexample of a device for processing a light radiation.

The device 100 shown in FIG. 1 comprises two reflective optical elements102 and 104 between them forming a multi-pass cavity 106 in which alight radiation to be processed 108 undergoes a plurality of reflectionsand propagations.

With a multi-pass cavity 106, the light radiation 108 encounters exactlythe same (no more, no less) intermediate optical elements (there is nointermediate element in the case of FIG. 1) reflecting and/ortransmitting it between each pair of successive reflections by means ofthe reflective elements 102, 104, i.e.:

-   -   for each path starting from a first reflective optical element        102 (from the at least two reflective optical elements) and        going to a second reflective optical element 104 (from the at        least two reflective optical elements), and    -   for each path starting from the second reflective optical        element 104 and going to the first reflective optical element        102, these intermediate optical elements preferably not        comprising any lens and/or any polarizing element (cube or        plate) changing the polarization of the light radiation 108.

The reflective element 104 comprises a through-hole 110 making itpossible for the optical radiation to enter the multi-pass cavity 106 tobe processed and to leave the cavity 106 after having been processed.

The reflective element 102 comprises a planar reflective surface 112 andthe reflective element 104 comprises a concave or curved reflectivesurface 114.

The multi-pass cavity 106 defined by the mirrors 102 and 104 is arrangedso that the light radiation 108 is reflected a plurality of times byeach of the mirrors, at different positions, in turn. Thus, in theexample shown, the plane mirror 102 reflects the optical radiation 108eight times, at eight different reflection positions on the planarsurface 112 and the curved mirror 104 reflects the optical radiation 108seven times, at seven different reflection positions on the surface 114.

The reflective element 104 is formed by a curved or concave mirror anddoes not apply any modification to the spatial phase profile of theoptical radiation 108 apart from its curvature.

The reflective element 102 is called corrective. This reflective element102 is formed by a plane mirror the reflective surface 112 of which isdeformed on the wavelength scale, applying a modification of the spatialphase of the optical radiation. To this end, the deformed plane mirrorhas an irregular surface at the level of each reflection position 116,so that each reflection position 116 is corrective and has a spatialphase profile modifying the spatial phase of the radiation 108. Thus,each reflection region/zone/position 1161-1168 has different depths forat least two spatial components of the radiation 108 and carries out amodification of the spatial phase of the optical radiation 108, i.e.,different phase shifts of at least two spatial components of theradiation 108.

Each reflective optical element 102, 104 is arranged in order to reflectthe light radiation 108 a plurality of times (preferably at least fourtimes, preferably at least six times).

The multi-pass cavity 106 is arranged so that the light radiation 108goes back-and-forth several times between the (at least) two reflectiveoptical elements 102, 104.

In the example shown in FIG. 1, the reflective element 104 is notcorrective. Alternatively, the reflective element 104 can also becorrective, at least for a part of the reflection positions on thisreflective element 104.

In the example shown in FIG. 1, the reflective element 102 is correctivefor each reflection position on this reflective element 102.Alternatively, the reflective element 102 can be corrective, for only apart of the reflection positions on this reflective element 102.

In the example shown in FIG. 1, all the corrective reflection positions116 are represented differently, i.e., with different spatial phaseprofiles. Alternatively, each corrective reflection position 116 canhave one and the same irregularity, i.e., a phase profile identical tothat of another corrective reflection position 116.

FIG. 2 is a diagrammatic representation of a second non-limitativeexample of a device for processing light radiation.

The device 200 shown in FIG. 2 comprises all the components of thedevice 100 in FIG. 1.

In the device 200, the reflective elements 102 and 104 are placed in twodirections, respectively, 202 and 204, perpendicular to each other,while in FIG. 1 they are arranged in one and the same direction or intwo directions parallel to each other. The multi-pass cavity 106obtained with the device of FIG. 2 is called angled.

The device 200 also comprises an intermediate mirror 206, placed facingthe reflective elements 102 and 104 at an angle of 45° with respect toeach of the directions 202 and 204. The role of the intermediate mirror206 is to direct the optical radiation 108 originating from one of thereflective elements 102 or 104 to the other one of the reflectiveelements 104 or 102.

The intermediate mirror 206 is a 99% reflective mirror. As a result,this mirror 206 allows 1% of the radiation 108 to pass, each time thelatter is reflected on this intermediate mirror 206.

With a multi-pass cavity 106, the light radiation 108 encounters exactlythe same (no more, no less) intermediate optical elements (intermediateelement 206 in the case of FIG. 2) reflecting and/or transmitting itbetween each pair of successive reflections by the reflective elements102, 104, i.e.:

-   -   for each path starting from the first reflective optical element        102 and going to the second reflective optical element 104, and    -   for each path starting from the second reflective optical        element 104 and going to the first reflective optical element        102, these intermediate optical elements preferably not        comprising any lens and/or any polarizing element (cube or        plate) changing the polarization of the light radiation 108.

FIG. 3 is a representation in the form of a diagram of a non-limitativeexample of a method according to the disclosure for designing a deviceaccording to the disclosure.

The method 300 comprises an initial step 302 of producing a multi-passcavity, for example, the cavity 106, by combining the two mirrors 102and 104.

The method then comprises a step 304 of characterizing the geometry ofthe measurements of the cavity, i.e., determining the geometricalcharacteristics of the cavity, of the reflection positions, etc. Such astep can be carried out by propagating a radiation in the cavity, forexample, the radiation to be processed.

In a step 306, the method determines the optimized phase profiles for atleast two corrective reflection positions on at least one of thereflective elements defining the multi-pass cavity. This step 306comprises at least one iteration of the following steps carried out foreach corrective reflection position concerned of each correctivereflective element:

-   -   a step 308 during which the radiation to be processed and the        reference radiation are propagated digitally (in opposite        directions) to the level of the corrective reflection position,        taking account of the phase profiles already calculated for the        other corrective reflection positions;    -   a step 310 during which the value of the relative phase between        the reference radiation and the radiation to be processed is        determined at the level of the reflection position; and    -   a step 312 digitally modifying the phase profile at the level of        the corrective reflection position in order to compensate the        relative phase between the reference radiation and the radiation        to be processed at the level of the reflection position.

Steps 308-312 are iterated as many times as necessary in order to obtainan optimized overlap value (spatial scalar product) of the radiations,for example, determined beforehand.

Iteration of these steps makes it possible to determine an optimizedphase profile for each corrective reflection position concerned, makingit possible to obtain an optimized correlation parameter between theradiation to be processed and a reference radiation.

During a step 314, carried out after step 306, one or more phase plates,comprising the optimized phase profile for each corrective reflectionposition, are printed on the reflective element concerned, for example,by etching of the reflective surface 112 of the mirror 102.

FIG. 4 is a diagrammatic representation of a non-limitative example of asystem according to the disclosure for designing a device according tothe disclosure.

The system 400 comprises a CCD camera 402 making it possible to measurethe radiation to be processed at the level of a plurality of reflectionpositions on a reflective element defining a multi-pass cavity, thesemeasurements making it possible at the same time to characterize theradiation to be processed as well as the geometrical properties of themulti-pass cavity.

A module 404 makes it possible, by simulation, to apply different phaseprofiles for each of the corrective reflection positions concerned so asto determine the optimized phase profile for each corrective reflectionposition.

Finally, a module 406 makes it possible to simulate the propagation ofthe radiation to be processed and of the reference radiation within themulti-pass cavity in the presence of the phase profiles provided by themodule 404, in order to determine the value of a correlation parameterbetween the radiation to be processed and the reference radiation indifferent corrective positions depending on:

-   -   measurements carried out by the CCD camera 402, in particular        the characterization of the radiation to be processed and the        geometry of the multi-pass cavity, and    -   spatial phase profiles provided by the module 404,    -   a virtual reference radiation, represented by a set of digital        data.

Depending on the correlation parameter determined by the module 406, thephase profile at the position considered in the module 404 is modified.

When the module 406 determines an optimized value for the correlationparameter, the phase profiles determined by the module 404 providingthis optimized value are stored in storage means 408.

These optimized phase profiles are then used in order produce/configureone or more phase plates, provided in order to be placed instead of thereflective element in question. Alternatively, it is possible to printthe phase plate or phase plates directly on the reflective element inquestion, as described with reference to FIG. 1.

In FIG. 4, the system 400 is shown in combination with the device 200 inFIG. 2. In this configuration, the CCD camera 402 is positioned behindthe intermediate mirror 206 and is focused on the reflective surface ofthe reflective element in question, namely the reflective surface 112 ofthe plane mirror 102.

However, it is also possible to use the system 400 in FIG. 4 to design adevice according to the configuration shown in FIG. 1. In thisconfiguration, the CCD camera 402 is positioned behind the reflectiveelement in question, namely behind the mirror 102.

Of course, the disclosure is not limited to the examples that have justbeen described. For example, in the given examples the correctiveelement is an optical element defining the multi-pass cavity.Alternatively or in addition, it is possible to have at least onecorrective optical element that is different from the reflective opticalelements defining the multi-pass cavity and placed between thesereflective elements, such a corrective optical element being an opticalelement reflecting or transmitting the optical radiation, such as, forexample, the intermediate mirror 206 in FIG. 2.

In addition, in the examples given, the corrective positions are allpositions reflecting the light radiation. Alternatively or in addition,it is possible to have at least one corrective position that carries outa transmission of the light radiation.

What is claimed is:
 1. A method for designing a phase plate having aplurality of corrective positions, each corrective position beingreflective and having an irregular surface such that a spatial phaseprofile of the corrective position has a different phase shift forseveral different reflection points of the corrective position, themethod comprising the following steps: propagating a light radiation anda reference radiation in a multi-pass cavity defined by a reflectiveoptical element and the phase plate, and in which the light radiationand the reference radiation travel back-and-forth at least four times,between at least four different reflection positions of the phase plate;optimizing an interference between the light radiation and the referenceradiation, the optimization comprising at least one iteration of thefollowing optimization steps, carried out for at least one correctiveposition: determining a parameter relating to the interference betweenthe light radiation and the reference radiation, and modifying thespatial phase profile of the at least one corrective position; andconfiguring the corrective position such that the corrective positionexhibits the phase profile determined during the optimization step. 2.The method of claim 1, wherein the optimization step of determining theparameter comprises determining a value of a difference in phase betweenthe reference radiation and the light radiation at the level of thereflection position.
 3. The method of claim 2, wherein configuring thecorrective position comprises modifying the phase profile of thecorrective reflection position to compensate for the difference in phasebetween the reference radiation and the light radiation.
 4. The methodof claim 1, wherein the optimization steps are iterated as many times asnecessary to obtain an optimized overlap value of the light radiationand the reference radiation.
 5. The method of claim 1, wherein the lightradiation and the reference radiation are propagated in the multi-passcavity by simulation.
 6. The method of claim 5, wherein the simulationtakes into account at least one optical or physical characteristic of atleast one optical element of the multi-pass cavity.
 7. The method ofclaim 6, wherein the at least one characteristic comprises a length ofthe cavity or angles between optical elements of the multi-pass cavity.8. The method of claim 6, wherein the at least one characteristic iscalculated by using information captured during a step of collectingdata relating to a geometry of the multi-pass cavity.
 9. The method ofclaim 1, wherein the reference radiation propagates in a direction ofpropagation opposite to a direction of propagation of the lightradiation.
 10. The method of claim 1, wherein the irregular surface hasspatial structures have a size at least five times smaller than thetotal size of the corrective position.
 11. The method of claim 1,wherein the light radiation is obtained at the outlet of a diffusingmedium, and the reference radiation is identical to the light radiationbefore the light radiation has passed through the diffusing medium. 12.The method of claim 1, wherein the multi-pass cavity does not includeany intermediate optical element between the reflective optical elementand the phase plate.
 13. A method for designing a phase plate having aplurality of corrective positions, each corrective position beingreflective and having an irregular surface such that a spatial phaseprofile of the corrective position has a different phase shift forseveral different reflection points of the corrective position, themethod comprising the following steps: collecting data relating to ageometry of a multi-pass cavity defined by a reflective optical elementand the phase plate and in which a light radiation travelsback-and-forth at least four times, between at least four differentreflection positions of the phase plate; simulating propagation of thelight radiation and a reference radiation in the multi-pass cavity; foreach corrective position, optimizing an interference between the lightradiation and the reference radiation to determine a phase profile ofthe corrective position that compensates for a difference in phasebetween the reference radiation and the light radiation; and configuringthe corrective position such that it exhibits the phase profiledetermined during the optimization step.
 14. The method of claim 13,wherein the reference radiation propagates in a direction of propagationopposite to a direction of propagation of the light radiation.
 15. Themethod of claim 13, wherein the irregular surface has spatial structureshave a size at least five times smaller than the total size of thecorrective position.
 16. The method of claim 13, wherein the multi-passcavity does not include any intermediate optical element between thereflective optical element and the phase plate.
 17. The method of claim13, wherein collecting data relating to the geometry comprisesdetermining geometrical characteristics of the multi-pass cavity or ofthe reflection positions.
 18. The method of claim 17, whereindetermining the geometrical characteristics of the multi-pass cavity orof the reflection positions comprises propagating a radiation in themulti-pass cavity.
 19. The method of claim 13, wherein collecting datarelating to the geometry comprises propagating a radiation in thecavity.